Field of the Invention
The present invention relates to an insulated synchronous rectification DC/DC converter.
Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., each operate receiving commercial AC electric power from an external circuit. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet PCs are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (AC/DC converter) that performs AC/DC conversion of commercial AC voltage. Alternatively, in some cases, such an AC/DC converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing an AC/DC converter 100r investigated by the present inventor. The AC/DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 200r. 
The commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise included in the commercial AC voltage VAC. The rectifier circuit 104 is configured as a diode bridge circuit which performs full-wave rectification of the commercial AC voltage VAC. The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106, thereby generating a converted DC voltage VIN.
An insulated DC/DC converter 200r receives the DC voltage VIN via an input terminal P1, steps down the DC voltage VIN thus received, and supplies an output voltage VOUT stabilized to the target value to a load (not shown) connected to an output terminal P2.
The DC/DC converter 200r includes a primary-side controller 202, a photocoupler 204, a feedback circuit 206, an output circuit 210, a synchronous rectification controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, a switching transistor M1, and a synchronous rectification transistor M2. The output circuit 210 has the same topology as those of typical synchronous rectification flyback converters, and accordingly description thereof will be omitted.
The switching transistor M1 connected to the primary winding W1 of the transformer T1 performs switching so as to step down the input voltage VIN, thereby generating the output voltage VOUT. With such an arrangement, the primary-side controller 202 adjusts the duty ratio of the switching of the switching transistor M1.
The output voltage VOUT of the DC/DC converter 200r is divided by means of resistors R1 and R2. The feedback circuit 206 includes a shunt regulator or an error amplifier that amplifies the difference between the divided voltage (voltage detection signal) VS and a predetermined reference voltage (VREF) (not shown), and generates an error current IERR that corresponds to the difference, which is drawn (as a sink current) via a light-emitting element (light-emitting diode) arranged on the input side of the photocoupler 204.
A feedback current IFB flows through a light-receiving element (phototransistor) on the output side of the photocoupler 204 according to the error current IERR that flows on the secondary side. The feedback current IFB is smoothed by means of a resistor and a capacitor, and is input to a feedback (FB) terminal of the primary-side controller 202. The primary-side controller 202 adjusts the duty ratio of the switching transistor M1 based on the voltage (feedback voltage) VFB at the FB terminal.
The synchronous rectification controller 300r switches on and off the synchronous rectification transistor M2 in synchronization with the switching of the switching transistor M1. The synchronous rectification controller 300r generates a pulse signal in synchronization with the switching of the switching transistor M1. For example, when the switching transistor M1 turns off, a pulse generator sets the pulse signal to a first state (e.g., high level) configured as an instruction to turn on the synchronization transistor M2. When the current IS that flows through the secondary winding W2 becomes substantially zero in an on period of the synchronous rectification transistor M2, the synchronous rectification controller 300r sets the pulse signal to a second state (low level) configured as an instruction to turn off the synchronous rectification transistor M2. The above is the overall configuration of the AC/DC converter 100r. 
As a result of investigating the switching converter 100r shown in FIG. 1, the present inventors have come to recognize the following problems.
In recent years, from the viewpoint of power saving, there is a great demand for such an AC/DC converter 100r having further reduced power consumption when it operates with a light load or otherwise operates without a load (which will be referred to as the “standby state”). In order to meet this demand, the DC/DC converter 200r operates in a so-called burst mode (which will also be referred to as the “PFM mode”) in the standby state. In the burst mode, the primary-side controller 202 switches on and off the switching transistor M1 once or otherwise several times such that the output voltage VOUT becomes greater than the target level, and suspends the switching of the switching transistor M1 until the output voltage VOUT drops to a lower limit level determined according to the target level of the output voltage VOUT. Such an arrangement reduces the electric power used to switch on and off the switching transistor M1 (the electric power required to charge and discharge the gate capacitance of the switching transistor M1, for example), thereby providing high-efficiency AC/DC conversion.
However, there appears to be no end to the demand for reducing power consumption, and so the DC/DC converter 200r is required to provide further reduced power consumption.